What's the difference?

The jam packed road on working days is like this!

I've recently changed my working time from 8:45am to 8am due to the horrendous jam on the roads these days.
I started out at 7:45am on Tuesday and arrived at 8:15am.
So then I started out at 7:40am and still arrived at 8:15am.
I started out at 7:35am this morning and still arrived at 8:15am!!!

What the........